EP2568417B1

EP2568417B1 - Antenna device and method of setting resonant frequency of antenna device

Info

Publication number: EP2568417B1
Application number: EP12188215.3A
Authority: EP
Inventors: Noboru Kato; Katsumi Taniguchi; Jun Sasaki; Naoki Gochi
Original assignee: Murata Manufacturing Co Ltd
Current assignee: Murata Manufacturing Co Ltd
Priority date: 2009-04-21
Filing date: 2009-09-18
Publication date: 2018-07-04
Anticipated expiration: 2029-09-18
Also published as: EP2424042B1; JP2011103702A; CN103022661A; CN102916248B; JP4687832B2; US20150171509A1; JP2012157071A; CN102405556B; CN102405557B; US9203157B2; EP2424041A4; JPWO2010122888A1; WO2010122685A1; CN105680148A; CN102916248A; JP5177312B2; JP2012039654A; CN105449341A; JP2012135020A; EP2424042A4

Description

Technical Field

The present invention relates to antenna devices used in RFID systems and short-range wireless communication systems, such systems performing communication with appliances that are communication partners through electromagnetic signals, and to methods of setting the resonant frequency of such antenna devices.

Background Art

In RFID systems and short-range wireless communication systems, which have become widely used in recent years, in order to facilitate communication between mobile electronic appliances such as mobile telephones or between a mobile electronic appliance and a reader/writer, a communication antenna is mounted in each of the appliances. Among such antennas, an antenna to be mounted in a mobile electronic appliance is disclosed in JP 2006-270681 A1 .
Fig. 1 is a view illustrating communication between a mobile information terminal 21 and a reader/writer as disclosed in JP 2006-270681 A1 when the mobile information terminal 21 and the reader/writer are in the vicinity of each other. In the example illustrated in Fig. 1, a magnetic field H, which is part of electromagnetic waves radiated from a transmitting/receiving antenna unit 26 of the reader/writer, is affected by metal objects such as a battery pack 25 within a main body 22 of the terminal and is weakened due to reflection, absorption and the like. A metal layer 30 is arranged closer to a side onto which the electromagnetic waves are incident than a communication surface CS of an antenna module 10 is. An induced current (eddy current) is generated due to the external magnetic field acting on the front surface of the metal layer 30 and a magnetic field H1 generated as a result of this current causes generation of an induced electric current in an antenna coil 15 of the antenna module 10.
In this example, the metal layer 30 is arranged close to and facing the antenna module 10 so as to cover part of the antenna coil 15 and thereby inductive coupling occurs between the transmitting/receiving antenna unit 26 of the reader/writer and the antenna coil 15 of the antenna module 10 via the magnetic field component H1 that arises around the metal layer 30.
The antenna device illustrated in Fig. 1 attempts to solve a problem in that when the antenna of the antenna device and the antenna on the communication partner side are brought close to each other and the distance therebetween becomes very small, the communication characteristics fluctuate greatly depending on the magnitude of positional displacement between the centers of the two antenna. In order to reduce the degree to which magnetic flux, which is attempting to link the antenna coil 15 of the antenna module 10 on the mobile information terminal 21 side and transmitting/receiving antenna unit 26 on the reader/writer side, is blocked by metal objects such as the battery pack 25 within the casing, the metal layer 30 is provided to induce magnetic flux in that region. Therefore, it is not necessarily the case that a substantial effect can be obtained by changing the positions of shielding members such as the battery pack 25.
Furthermore, it is not necessarily the case that the metal layer 30 effectively acts to increase the communication range in a state in which the antenna device and the antenna on the communication partner side are spaced apart from each other.
US 6,452,563 B1 describes an arrangement for an antenna having the general shape of a loop, directly in the proximity of at least a metallic element. Each metallic element comprises an orifice arranged substantially opposite the surface defined by the antenna, and a slot forming a gap width, arranged through the thickness of the metallic element, between the internal edge delimiting the orifice and the external edge of said metallic element.
JP H 11 25244 A describes a non-contact data carrier package having a long communication distance. The non-contact data carrier package includes a conductive sheet arranged in the surrounding of an antenna coil so as to be overlapped on one part of the radiating face of the antenna coil.
WO 89/07347 A1 describes concentrator adapted to concentrate signals for reception by a receiver means, said concentrator comprising a conductive portion juxtaposed said receiver means, said concentrator being aligned so as to receive said signals and cause further signals to impinge on said receiver means, and thereby enhance reception of said signals by said receiver means.
EP 1 445 730 A1 describes a communication-capable electronic device which includes a metallic housing to accommodate electronic parts of the device, and a communication terminal that makes communications with a communication-cable IC card incorporating an antenna.
EP 1 484 816 A1 describes a soft magnetic member provided between an antenna coil and a metal surface of a conductive member such as a metal casing.

Summary of the invention

It is an object of the invention to provide an antenna device that, even when the size of the antenna device has been made small compared to that of an antenna on the communication-partner side, is capable of stable communication and is also capable of increasing the maximum possible communication range.
This object is achieved by an antenna device according to claim 1.

Advantages of Invention

According to the invention, a current flows through the conductor layer so that a magnetic field, which is generated by the current flowing through the coil conductor, is blocked. Then, a current flows around the periphery of the opening in the conductor layer and a current flows along the periphery of the slit and around the periphery of the conductor layer due to the cut-edge effect. As a result, a magnetic field arises around the conductor layer and the communication range can be increased.
In addition, since the conductor layer causes large loops of magnetic flux to circulate, the magnetic flux reaches from the antenna device to the antenna on the communication partner side or from the antenna on the communication partner side to the antenna device, and the maximum possible range of communication between the antenna device and the antenna on the communication partner side becomes large.

Brief Description of Drawings

Fig. 1 is a view illustrating a situation in which communication is performed between the mobile information terminal 21 and the reader/writer as disclosed in JP 2006-270681 A1 when the mobile information terminal 21 and the reader/writer are in the vicinity of each other.
Fig. 2(A) is a view of a rear face of an electronic appliance that is equipped with an antenna device according to a first example.
Fig. 2(B) is a plan view of the interior of a lower casing viewed from the rear face side.
Fig. 3(A) is a plan view of the antenna coil module 3 and Fig. 3(B) is a front view thereof.
Fig. 4(A) is a sectional view illustrating a state in which the antenna device 101 and the antenna on the reader/writer side are magnetically coupled. Fig. 4(B) is a sectional view illustrating a state in which there is no conductor layer 2, as a comparative example.
Figs. 5 are views of an antenna device according to a second example.
Fig. 5(A) is a plan view of the interior of a lower casing 1 of an electronic appliance. Fig. 5(B) is a plan view of a state in which the antenna coil module 3 has been mounted on an inner face of the lower casing 1.
Figs. 6 are plan views of an antenna device 103 according to an embodiment. Specifically, Fig. 6(A) illustrates the current that flows through a coil conductor 31 and Fig. 6(B) illustrates a current 1 that flows through a conductor layer 2.
Fig. 7(A) is a plan view of an antenna coil module 13 according to a third example and Fig. 7(B) is a front view thereof.
Fig. 8 is a plan view of an antenna device 104 according to a fifth embodiment.

<<First Example>>

An antenna device and a method of setting a resonant frequency of the antenna device according to a first example will be described with reference to Figs. 2 to 4.
Fig. 2(A) is a view of a rear face of an electronic appliance that is equipped with the antenna device according to the first example. The rear face of the electronic appliance faces an antenna on a reader/writer side, which is a communication partner side. Fig. 2(B) is a plan view of the interior of a lower casing on the rear face side. Only the structures of principal components are illustrated in Figs. 2(A) and 2(B).
As illustrated in Fig. 2(A), a conductor layer 2 is formed on an outer face of a lower casing 1. The conductor layer 2 is for example a vapor-deposited metal film such as an aluminum film. A conductor opening CA and a slit SL, which connects the conductor opening CA and an outer edge of the conductor layer 2, are formed in the conductor layer 2.
As illustrated in Fig. 2(B), an antenna coil module 3 is arranged on an inner face of the lower casing 1 so as to partially cover the conductor opening CA.
In this example, a camera module is to be installed in the conductor opening CA. It is necessary for the lens of the camera module to be exposed to the outside through an opening in the casing. Therefore, by arranging the conductor opening so as to coincide with the opening provided in the casing in this way, there is no need to design a special casing taking into account provision of the conductor layer 2.
In addition, in cases such as where part of an outer face of the casing is made to be metallic from a design point of view, a metal film is formed on the outer face of the casing by vapor deposition or the like and the metal film may double as the conductor layer.
By forming the conductor layer 2 on an inner or outer face of the casing of the electronic appliance in this way, there is no need to ensure there is a dedicated space in which to arrange the conductor layer 2 and a conductor layer 2 having a large area can be arranged.
Fig. 3(A) and 3(B) are a plan view and a front view of the antenna coil module 3, respectively. The antenna coil module 3 is equipped with a rectangular-sheet-shaped flexible substrate 33 and a magnetic sheet 39 having the same rectangular sheet-like shape as the flexible substrate 33. Connectors 32 are formed on the flexible substrate 33 and are used for connecting a spiral-shaped coil conductor 31, in which a winding center portion serves as a coil opening CW, and an external circuit. The magnetic sheet 39 is for example composed of a ferrite formed in a sheet-like shape.
The number of windings (number of turns) of the coil conductor 31 is determined on the basis of the required inductance. If there is only one turn, a simple loop-shaped coil conductor is formed.
The antenna coil module 3 is arranged so that the antenna coil module 3 is mounted on an inner face of the lower casing 1 of the electronic appliance as illustrated in Fig. 2(B), and when an upper casing is stacked thereon the connectors 32 contact predetermined pins of a circuit board mounted in the upper casing.
The circuit board is provided with a capacitor that is connected in parallel with the connectors 32. The resonant frequency is determined by the inductance defined by the coil conductor 31 and the magnetic sheet 39 of the antenna coil module 3 and the capacitance of the capacitor. For example, in the case where a HF band is used having a central frequency of 13.56 MHz, the resonant frequency is determined to be 13.56 MHz.
Fig. 4(A) is a sectional view illustrating a state in which the antenna device 101 and the antenna on the reader/writer side are magnetically coupled. Furthermore, Fig. 4(B) is a sectional view illustrating a state in which there is no conductor layer 2, as a comparative example. Components of the casing of the electronic appliance are omitted from the figures.
Since at least part of the coil conductor 31 of the antenna coil module 3 is covered by the conductor layer 2, a current flows through the conductor layer such that the magnetic field that arises due to the current flowing through the coil conductor is blocked. Then, a current, which flows around the periphery of the opening of the conductor layer, passes along the periphery of the slit and flows around the periphery of the conductor layer due to the cut-edge effect. In addition, a current also flows through the planar portion of the conductor layer. Accordingly, a magnetic field arises around the conductor layer 2 and since magnetic flux does not penetrate through the conductor layer 2, magnetic flux MF attempts to bypass the conductor layer 2 along a path in which the conductor opening CA of the conductor layer 2 is on the inside and the outer edge of the conductor layer 2 is on the outside. As a result, the magnetic flux MF draws relatively large loops that link the inside and the outside of a coil conductor 41 of an antenna 4 on the reader/writer side. In other words, the antenna device 101 and the antenna 4 on the reader/writer side are magnetically coupled.
Furthermore, the magnetic flux, which is attempting to link the coil conductor 31 and the antenna on the communication partner side, can circulate through the coil opening CW and the conductor opening CA due to the fact that the coil opening CW and the conductor opening CA at least partially overlap when the coil conductor 31 is viewed in plan view. Specifically, when the coil opening CW and the conductor opening CA overlap over almost the entire peripheries thereof when the coil conductor 31 is viewed in plan view, the coil conductor 31 can be made to effectively radiate a magnetic field.
In addition, large magnetic field loops can be made to arise due to the fact that the area of the conductor layer 2 is larger than the area of the region over which the coil conductor 31 is formed.
Furthermore, the directivity of the antenna can be made to be wider in the antenna direction of the communication partner by stacking the magnetic sheet 39 on the face of the flexible substrate 33 that is on the side farther from the antenna on the communication partner side.
In this way, since it turns out that the magnetic flux is oriented in directions that extend beyond the conductor layer 2, stable communication can be realized even when the antenna device 101 and the reader/writer are close to each other in a state where the center of the antenna device 101 (center of antenna coil module 3) and the center of the antenna 4 of the reader/writer are aligned.
Furthermore, since conductor loops are not formed in the conductor layer 2 due to the fact that the conductor layer 2 is provided with the slit SL, as illustrated in Fig. 4(A), when the magnetic flux MF circulates through the conductor opening CA and around the outer edge of the conductor layer 2, eddy currents can be prevented from arising in the conductor layer 2 and losses due to such eddy currents can be suppressed. As a result, the maximum possible communication range can be secured between the antenna device 101 and the antenna 4 on the reader/writer side.
In contrast to this, as illustrated in Fig. 4(B), when there is no conductor layer 2, the magnetic flux MF, which passes through the coil opening CW, does not spread out in the left-right direction in Fig. 4(B)(direction of spreading when the antenna module 3 is viewed in plan view) and the degree of coupling with the antenna 4 on the reader/writer side is small. Therefore, a phenomenon occurs in which communication becomes increasingly unstable the closer the antenna device 101 and the reader/writer are brought to each other.
Moreover, in the antenna device of the invention, the conductor layer causes large loops of magnetic flux to circulate and therefore magnetic flux reaches from the antenna device to the antenna on the communication partner side or from the antenna on the communication partner side to the antenna device and the maximum possible range of communication between the antenna device and the antenna on the communication partner side becomes large.

The following table lists results obtained when the size of the antenna coil module 3 was varied and the range over which communication can be performed between an antenna device 101 equipped with each of the antenna coil modules 3 and the antenna on the communication partner side was measured.

Size of Antenna Coil Module	Without Conductor Layer	With Conductor Layer
22.5 mm x 20 mm	0 - 24 mm	0 - 44 mm
22.5 mm x 19 mm	0 - 23 mm	0 - 43 mm
22.5 mm x 18 mm	0 - 19 mm	0 - 41 mm
22.5 mm x 17 mm	-	0 - 39 mm
22.5 mm x 16 mm	-	0 - 38 mm

For example, in the case of the antenna device equipped with the antenna coil module 3 having a size of 22.5 mm x 18 mm, when the conductor layer is included, the maximum possible communication range is increased to 41 mm, in contrast to when the conductor layer is not provided and the maximum communication range is 19 mm. Furthermore, for example, in the case of the antenna devices equipped with the antenna coil modules 3 having sizes equal to or less than 22.5 mm x 17 mm, communication is not possible at any distance when the conductor layer is not provided. In contrast, when the conductor layer is included, communication is possible over a wide range from 0 mm to 38 mm even in cases where the antenna device is configured so as to be equipped with a very small antenna coil module 3 such as the one having a size of 22.5 mm x 16 mm.

Setting of the central resonant frequency of the antenna device 101 is performed as follows. In a state in which the antenna coil module 3 illustrated in Fig. 2(B) is not in the vicinity of the conductor layer 2, in other words, in a stand alone state in which the antenna coil module 3 has not yet been mounted in the lower casing 1, while the antenna coil module 3 is connected in parallel with a resonance capacitor, the resonant frequency is set in advance to be lower than the central resonant frequency of the frequency band to be used. As illustrated in Figs. 2(B) and 4(A), when the antenna coil module 3 is brought close to the conductor opening CA of the conductor layer 2, since the inductance value of the antenna coil module 3 becomes smaller, the resonant frequency of the antenna device 101 increases. Consequently, the inductance value of the antenna coil module 3 in the stand alone state is determined so that the resonant frequency of the antenna device 101 once the antenna device 101 has been constructed by mounting the antenna coil module 3 inside the casing of the electronic appliance approximately coincides with the central resonant frequency of the frequency band to be used.
In addition, the inductance of the antenna device varies in accordance with the length and width of the slit SL. For example, when the slit SL is made to be large, the degree of reduction of the inductance value (increase of resonant frequency) when the conductor layer 2 is brought close to the antenna coil module 3 can be suppressed. Accordingly, the length and width of the slit SL are set so as to obtain a desired inductance value.
Furthermore, since the inductance varies with the positional relationship between the antenna coil module 3 and the conductor layer 2 and with the shape and size of the conductor opening CA of the conductor layer 2, the positional relationship between the antenna coil module 3 and the conductor layer 2 and the shape and size of the conductor opening CA of the conductor layer 2 are set so as to obtain a desired inductance value.
In this way, since the loops of magnetic flux are large due to the presence of the conductor layer, the coil opening CW of the coil conductor 31 may be made small and the antenna coil module can be reduced in size. Furthermore, together with making the coil opening CW small, the number of coil turns can be increased and the number of values of the inductance of the antenna coil module that can be adopted can be increased. As a result, setting of the resonant frequency is simplified.
As described above, characteristics are used for which a magnetic field cannot penetrate into the conductor layer and the emission pattern of the magnetic field at the frequency being used is caused to change due to the presence of the conductor layer.
Stable communication can be performed with the antenna device 101 of the invention, even when there are various sizes of the antenna device 101 and the antenna 4 on the reader/writer side. In other words, in the case where the antenna 4 on the reader/writer side is large, magnetic flux circulates in large loops due to the presence of the conductor layer 2 and the magnetic flux from the antenna device 101 reaches the antenna 101 on the reader/writer side, and in the case in which the antenna 4 on the reader/writer side is small, the distance that the magnetic flux reaches is increased due to the presence of the conductor layer 2.

<<Second Example>>

Figs. 5 are views of an antenna device 102 according to a second example. Fig. 5(A) is a plan view from the inner face side of the lower casing 1 of the electronic appliance. Furthermore, Fig. 5(B) is a plan view of a state in which the antenna coil module 3 has been mounted on an inner face of the lower casing 1.
In the second example, the conductor layer 2 is provided on an inner face of the lower casing 1. The conductor layer 2 is formed by vapor deposition of a metal film such as aluminum or by adhesion of a metal foil. In this way, a conductor layer may be provided on an inner face of the casing.
Moreover, in the example illustrated in Fig. 5, the slit SL of the conductor layer 2 is provided in a portion for which the distance from the conductor opening CA to the edge is short.
The conductor layer 2 may be connected to the ground of a circuit inside the electronic appliance. By doing this, the conductor layer 2 can also serve as a conductor layer for shielding the electronic appliance.

<<Embodiment of the invention>>

Figs. 6(A) and 6(B) are plan views of an antenna device 103 according to the embodiment. Fig. 6(A) illustrates an electric current that flows through the coil conductor 31 and Fig. 6(B) illustrates an electric current 1 that flows through the conductor layer 2. The antenna device 103 is equipped with the antenna coil module 3 and the conductor layer 2. The antenna coil module 3 is constructed by stacking the flexible substrate on which the spiral-shaped coil conductor 31 has been formed on top of a magnetic sheet. Basically, this is the same as the configuration illustrated in Fig. 3. However, in this embodiment, the two terminals of the coil conductor extend from the flexible substrate and connectors are provided at positions separated from the coil conductor 31.
The conductor layer 2 is provided with the conductor opening CA and the slit SL, which connects the conductor opening CA and the outer edge of the conductor layer 2.
When the coil conductor 31 and the conductor layer 2 are viewed in plan view, the coil opening CW and the conductor opening CA are aligned and overlap over almost the entire peripheries thereof. With such a configuration, when the coil conductor 31 is viewed in plan view, the entirety of the coil conductor 31 can be made to be covered by the conductor layer 2. Consequently, since the magnetic flux generated by the coil conductor 31 attempts to totally link to the conductor layer 2, a large current is generated in the conductor layer 2 in a direction opposite to that of the current that flows through the coil conductor 31 so as to block this magnetic flux. The large electric current I, which flows around the periphery of the conductor opening CA, flows along the periphery of the slit SL, through the plane of the conductor layer and along the periphery of the conductor layer due to the cut-edge effect. Accordingly, a strong magnetic field can be generated by the conductor layer 2 and the communication range can be further widened. In addition, the loops of magnetic flux passing through the conductor opening CA and the coil opening CW and circulating around the conductor layer 2 are more effectively widened. Consequently, stable communication can be performed even when the electronic appliance equipped with the antenna device 103 is oriented in any direction among in-plane directions relative to the surface of the antenna on the reader/writer side.

<<Third Example>>

Fig. 7(A) and Fig. 7(B) are a plan view and a front view of an antenna coil module 13 according to a third example, respectively. The antenna coil module 13 is positioned so as to be in the vicinity of a conductor layer, as illustrated in Fig. 2, similarly to as in the case of the first example. Due to the antenna coil module 13 being positioned in this way, a large electric current is generated in the conductor layer 2 and a strong magnetic field is generated around the conductor layer 2.
The antenna coil module 13 is provided with the rectangular-plate-shaped flexible substrate 34 and the magnetic sheet 39 having the same rectangular shape. The spiral-shaped coil conductor 31, in which a winding center portion serves as the coil opening CW, and connectors 32, which are used for connecting to an external circuit, are formed on the flexible substrate 34. The coil conductor 31 is formed over two layers of the flexible substrate 34. The two layers are connected by via conductors. The magnetic sheet 39 is for example composed of a ferrite formed in a sheet-like shape.
In this way, a strong magnetic field is generated by the coil conductor due to the coil conductor 31 being wound over a plurality of layers, and as a result, a large current is made to flow in the conductor layer. In addition, the coil conductor can be made to center on such a position as to be aligned with the inner edge of the conductor opening. Accordingly, magnetic flux, which is centered on and generated by the coil conductor 31 attempts to link to the conductor layer and therefore a large current can be made to flow in the conductor layer.

<<Fourth Example>>

Fig. 8 is a plan view of an antenna device 104 according to a fourth example. The antenna device 104 is provided with the antenna coil module 3 and the conductor layer 2. In the antenna coil module 3, a flexible substrate on which a spiral-shaped coil conductor has been formed is stacked on top of a magnetic sheet. This configuration is similar to that illustrated in Fig. 6. However, in this example, the coil opening CW is formed so as to be slightly smaller than the conductor opening CA.
Since part of the coil conductor 31 is covered by the conductor layer 2, a current flows through the conductor layer 2 due to the magnetic flux generated by the coil conductor 31 as a result of this structure.
In addition, in each of the above-described examples, the conductor layer is formed on an outer face or an inner face of the casing, but the conductor layer may be arranged on an internal component of the casing. Furthermore, in the case where the casing itself is metal, the casing may be used as the conductor layer.

CA: conductor opening
CW: coil opening
MF: magnetic flux
SL: slit
1: lower casing
2: conductor layer
3, 13: antenna coil module
4: antenna on reader/writer side
31: coil conductor
32: connector
33, 34: flexible substrate
39: magnetic sheet
41: coil conductor
101 - 104: antenna device

Claims

An antenna device (103) comprising:
an antenna coil module (3) including a coil conductor (31) having a loop shape or a spiral shape in which a winding center portion defines a coil opening (CW); and

a conductor layer (2) including a conductor opening (CA) and a slit (SL) connecting the conductor opening (CA) and an outer edge of the conductor layer (2);

wherein the conductor layer (2) is superposed on top of the coil conductor (31) of the antenna coil module (3), and is larger than an area of a region in which the coil conductor (31) is disposed;

characterized in that the antenna coil module (3) further comprises a magnetic layer (39) that is arranged along the coil conductor (31); wherein

the magnetic layer (39) is arranged on a side of the coil conductor (31) that is farther from the conductor layer (2) than the coil conductor (31); and

when the coil conductor (31) is viewed in plan view, the distance between the outer edge of the conductor layer (2) and the outer edge of the coil conductor (31) is larger than the distance between the edge of the conductor opening (CA) and the inner edge of the coil conductor (31), and the whole coil conductor (31) is arranged in the area between the edge of the conductor opening (CA) and the outer edge of the conductor layer (2).
The antenna device according to claim 1, wherein the coil conductor (31) is wound across a plurality of layers.
The antenna device according to claim 1, wherein, when the coil conductor (31) is viewed in plan view, the coil opening (CW) and the conductor opening (CA) overlap each other over entire peripheries thereof.
The antenna device according to claim 1, characterised in that the antenna coil module (3) further comprises a substrate (33), wherein the coil conductor (31) is located on the substrate (33), the magnetic layer (39) being stacked on a surface of the substrate (33) opposite to a surface of the substrate (33) on which the coil conductor (31) is located.
An electronic appliance comprising:
a metal casing (1) ; and
the antenna device (103) according to claim 1; wherein the antenna coil module (3) is provided on an inner surface of the
metal casing (1); and
the metal casing defines the conductor layer (2).
An electronic appliance comprising:
the antenna device (103) according to claim 1; and

a casing (1); wherein

the conductor layer (2) is located on an inner surface or an outer surface of the casing (1); and

the antenna coil module (3) is provided in the casing.